1. Technical Area
The invention relates to an arrangement for a manufacturing crystalline materials, in particular to those having large dimensions. The invention relates also to manufacturing semiconductors according to the VB-method (Vertical Bridgman), or according to the VGF-method (Vertical Gradient Freeze).
2. Description of Related Art
The economy of manufacturing crystals is substantially dominated by the throughput, i.e., crystallized mass per time unit, of the material through the crystallization furnace as well as dominated by the consumption of energy which is necessary therefore. Many single or polycrystalline materials are frozen out of the melt being directed in a gradient temperature field. Such methods are called gradient-freeze- or Bridgman-method, see also Wilke, K.-Th. and J. Bohm in: “Kristallzüchtung”, Ed. K.-Th. Wilke, First Edition, VEB Deutscher Verlag der Wissenschaften, 1973, 1-922.
For many material characteristics of the crystal to be manufactured such as the integratin of dopants, the structural perfection or the elastic stress, it is particularly important that the crystallization front in the crucible does not reveal any or only small curvatures during the freeze. In order to avoid curvatures of the phase interfaces, each point in a direction perpendicular to the direction of the freeze must preserve the axial heat flux condition:v=(qmelt−qcrystal)/Δhlatentρcrystal.
Therein, v denotes the axial crystallization velocity, q denotes the axial heat flux in the melt and the crystal at the position of the crystallization front, respectively, Δhlatent denotes the specific latent heat of the phase transition and ρcrystal denotes the mass density of the frozen material. The heat flux in the crystal is determined as the product of the thermal conductivity λ and the temperature gradient grad(T). By approximation, the quantity ΔT/Δx may be specified, if x is the axial coordinate in the direction of freeze. By the approximation of a vanishing heat flux within the melt, qmelt=0, the growth velocity has an upper limit, which may be calculated by:v≦(λ/Δhlatentρcrystal)·grad(T).
The temperature gradient may, however, be selected for many materials such as not to attain arbitrarily large values due to the appearance of thermo-elastic stress. In particular, the growth velocity becomes necessarily smaller for crystals attaining larger lengths. In this case, the heat resistance of the crystal becomes larger with continuing crystallization and the latent heat power which can be dissipated, i.e., the latent heat per time unit, decreases. This again limits the growth velocity for long crystals.
For economical reasons, particularly of the material throughput in crystallization furnaces, it is thus less expedient to increase the length of the crystals beyond limits which depend on the material employed. On the contrary, the material throughput might be increased by expanding the area, on which the material is simultaneously crystallized. Therein, the latent heat power becomes larger proportional to the crystallization area, but the dissipation of heat from the phase interface equivalently increases in proportion to the area.
Often there are required only specific standardized material quantities for further processing the crystalline material to yield electronic components or integrated circuits, solar cells, optical components, etc. Hence, an arbitrarily large crystallization area in the crucible becomes out of question due to the lack of a corresponding demand.
It is also known to manufacture a number of comparatively thinner crystals adapted in shape in parallel to each other rather than crystallizing a larger block of material. Such Bridgman-like methods are known for optical materials such as rare earth-fluorides according to U.S. Pat. No. 3,796,552 A or for szintillator crystals such as barium fluoride according to EP 0,130,865 A1. Therein, however, only small ratios of the diameter with respect to the corresponding length of the crystals are employed.
For comparatively larger crystals, which are employed in micro- and opto-electronic, photovoltaic or optical applications, corresponding solutions are not aimed at until now. Namely, there is a large challenge to form the temperature field at each time step of the crystallization such that important characteristics as for example electric resistance, mobility of charged particles, point defect concentrations, structural perfection, elastic stress, refraction index, reflectivity, etc. meet the target requirements of the manufacturers with concern to the respective applications.
For semiconductor crystals, which are processed to semiconductor wafers during the further production process, the radial homogeneity plays a particularly important role for the processing of the components. For example, parameters such as specific electrical resistance, charged particle mobility, concentrations of dopants or foreign particles or the residual stress are to be distributed over the wafer with particular homogeneity.
With regard to single crystalline materials there have to be considered even stronger requirements with concern to the specific crystal orientation. Mostly, a seed crystal is furnished, which then determines the crystal direction of the growing crystal. Methods, in which the single crystal is generated by spontaneous seed formation followed by a seed selection, often dropout for material economic reasons.
The difficulty to germinate at a seed crystal is to form a temperature field, such that the melted material contacts the seed and the seed is then slightly melted back. It is convenient for Bridgman- or gradient-freeze-methods to select the arrangement of heat sinks and heat sources such that the desired temperature field can be generated. Proceeding to continuously larger constructions it becomes now necessary to arrange so-called liners with corresponding thermal characteristics, such that a temperature field can be formed extending over larger areas such as described in EP 1,147,248 or U.S. Pat. No. 6,712,904.